Servosystem adapted for automatic adjustment of radio transmitters



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IN VEN TOR. VINCENT R. BELONG Arrontvzy United States. PatentSERVOSYSTEM ADAPTED FOR AUTOMATIC ADJUSTMENT OF RADIO TRANSMITTERSVincent R; De Long, Cedar Rapids, Iowa, assignor to Collins RadioCompany, Cedar Rapids, Iowa, a' corporation 0E Iowa Application January31, 1957, Serial No. 637,567

11 Claims. (Cl. 318-28) 'Ifhisinvention'pertains to radio transmitterp'ower amplifier stages and more particularly to controlcircuits inservo systems for automatically adjusting the loading of I the stages.

An example of the application of servo systems to antomatic' loading isdescribed in United States Patent No. 2,376,667fissued to O. B.Cunningham et"al., on May 22, 1945. It' is wellknown that the exciterportion of a radio transmitter may be readily'tuned by a singlecont'rolwhich may be operated either automatically or manually. The varioustuning elements of an exciter may be'coupled"mechanically to'a singleshaft and a partictular position'of the shaft will correspond repeatedlyto a particular frequency to which the transmitting circuits are to betuned; However, since variations" in reactive and loadingcharacteristics of an'antenna are a function of" frequency, individualcontrol of reactive elements is requiredfor tuning the plate circuit ofthe final stage and "for coupling the antenna. Tuning and loading thefinal stage is' complicated further in that differentantennas may beused under different conditions or that a. particular antenna may havevarying characteristics because of changes in position.

In order to provide required'flexibility in tuning and loading ofthefinal stage, atransmitter may include a. servo system that is responsiveto phase changes fortuning the plate circuit anda servosystem' that isre-'- sponsive to plate current'changes for varying the couplingbetween-the'antennaand the plate circuit. These systems operatesatisfactorily when the power amplifier stage'is operated consistentlyin a particular type of serv-- ice; for example, class C service inwhichthe plate 'current'flows for less than'one-half ofeachcycle oftheimpressed alternating current signal; However, a com-- bination'ofvoltage control-and current control is re-- quiredfo'r the loadingservowhen the'final'sta'ge is to be operated over a wide range of platecurrent conditions.

An object of the presentinventionis' to supply ir'nproved controlcircuits which apply required control voltages to loading servo systemsfor maintaining constant the output impedance of transmitters fordifferent classes of service.

A-*furth"er object is to provide, in combination with loading controlcircuits, lock-out circuits which effectively disconnect loading controlcircuits during tuning of the final stages.

And still another object is to provide coarse-tuning control circuitsthat adjust output coupling circuits in responseto tuning of thefinal'staiges.

The following 'descriptionand appended 'claim'sma'y be more readilyunderstood with reference to the single drawing which shows in a circuitschematic diagram a final power amplifier stage utilizing the servoloadingcontrol circuits of this invention.

A power amplifier stage utilizing the control circuits of this inventionincludes'an electroncontrol device, first adjustable reactive elementsfor tuning the' output circuit to resonance, and second adjustablereactive ele- ICE ments for coupling the load to the output circuit ofthe stage. Although the function of one set of reactive elements isnamely for tuning and the function of the other set of reactive elementsis namely for loading, it is to be understood that the elements reactmutually and that when the value of either one is changed to obtainadesired change in its respective function, the other function is changeda lesser amount.

In the instant example, the automatically tuned amplifier stage includestriode electron tube 11 having a plate circuit which includes elements12 and 13 for tuning the circuit and elements 14 and 15 for couplingantenna 16 to'the plate circuit. Electron tube 11 may be a usual triodewhich includes cathode 17, control grid 18, and anode or plate 19. Inputlead 20 of the stage is connected between control grid 18 anddriveramplifier tun-' ing circuits of a preceding exciter. The cathode 17 isconnected through variable cathode resistor 21 to ground. Plate 19 isconnected through radio-frequency choke 22 to a source of B-plus voltageand also through blocking capacitor 23 to the plate tuning circuit. Theplate tuning. circuit that includes variable capacitor 13 and variableinductor 12 is a reactive L-type network and is connected to anotherreactive L-type network that includes variable capacitor 15 and variableinductor 14 for coupling antenna 16 to the plate circuits.

A servo system for simultaneously adjusting tuning ca pacitor 13 andvariable inductor 12 includes serially connected phase detector 24;servo amplifier 25, and servo motor 26. Servo motor 26 is mechanicallycoupled to inductor 12, capacitor 13, rate generator 27, and to acoarse-tuning control potentiometer 31. Phase detector 24 has one inputcircuit connected to the input or grid circuit of tube 11 and hasanother input circuit connected to the output circuit of the tube. Inresponse to departure in phase in the grid and plate circuits from 180,phase detector 24 develops a control voltage for application to servoamplifier 25. Servo motor 26 operates'to'adjust plate tuning elements 12and 13 to restore phase difference of l. Rate generator 27 is operatedby servo motor 26 to apply voltage through transformer 28 to acoarse-tuning bridge circuit and to a fine-tuning lock-out controlcircuit.

The servo system, which varies the coupling between the antenna and theplate circuits to control loading of the transmitter, includes servoamplifier 29 and servo motor 30L Servo amplifier 29 is connected tomotor 30 to control its operation and motor 30 is connected to variablecapacitor 15, variable inductor 14, and coarsetuning potentiometer 32.

The control circuits for servo amplifier 29 include a fine-loadingcontrol circuit, a coarse-loading control circuit, and an electricallockout circuit which disconnects tli'e'fine-loading control circuitfrom servo amplifier 29 while the coarse-loading control circuit isfunctioning. The coarse-loading control circuit includes coarse-tuningpotentiometers 31 and 32 that are connected in a bridge circuitarrangement with resistors 33 and 34. During plate tuning,alternating-current voltageis applied from rate generator 27 to theprimary winding of transformer 28. The secondary circuit of transformer28 includes diode rectifier 35 connected in series with filter capacitor336 A diagonal of the coarse-loading control bridge that includesotentiometers 31 and 32 is connected across :filter capacitor 36 so thatdirect-current voltage is applied :across the Potentiometers. Thejunction of resistors 33 and '34 that are connected in series betweenthe arms of- Potentiometers 31 and 32 is connected to the input circuitof servo amplifier 29.

The fine-loading control circuit is connected for comparing voltages atthree points in the amplifier stage. 'In general, the control circuitincludes two rectifying and smoothing circuits connected respectively tothe grid and plate circuits of the power amplifier stage and acombination rectifying and switching circuit connected to the cathodecircuit. The output circuits of the three rectifying or voltage-sensingcircuits are connected to a voltage divider wherein the output voltagesfrom the sensing circuits are combined to develop a control voltage ofrequired magnitude and polarity for application to the input circuit ofservo amplifier 2%.

The voltage-sensing circuit connected to plate 19 of tube 11 includescapacitors 55 and 56 that are connected in series between the plate andground. A conventional rectifier circuit is connected across capacitor56. This circuit includes radio-frequency choke 37, diode rectifier 38,filter capacitor 39, and potentiometer or load resistor 48. The arm ofpotentiometer 4'!) is connected to one end of the voltage divider whichsupplies voltage to the input of servo amplifier 29. The polarity ofdiode 38 is such that current flows in that direction required todevelop with respect to ground a negative voltage across potentiometer40.

The voltage-sensing circuit that is connected to grid 13 of tube 11includes capacitors 41 and 42 connected in series between the grid andground. A rectifier circuit similar to that for the platevoltage-sensing circuit is connected across capacitor 42. This circuitincludes radiofrequency choke 43, rectifier 44, filter capacitor 45, andpotentiometer 4-6. The polarity of diode 44 is such that voltagedeveloped with respect to ground across the potentiometer 46 ispositive. The arm of potentiometer .6 is connected to the voltagedivider forcomparing the positive voltage developed across potentiometer46 with the negative voltage developed across potentiometer 40.

The Voltage divider includes resistors 47, 48, and 49 connected inseries between the arms of potentiometers 40 and 46. Voltage for servoamplifier 29 is obtained from the junction of resistors 48 and 49 whichis connected to the servo amplifier through a lock-out circuit. Thevalues of the resistors of the voltage divider and the settings of thearms of potentiometers 44) and 46 are such that the control voltage atthe junction of resistors 48 and 4-9 becomes Zero for a predeterminedratio of voltage on plate 19 to voltage on grid 18.

A third voltage-sensing circuit becomes effective when the grid driveexceeds the maximum value that allows linear operation of the poweramplifier stage. This voltage-sensing circuit, which may be termed aclamping circuit, includes diode rectifier 50 which is connected betweencathode 17 of tube 11 and the junction of resistors 47 and 4-8 of thevoltage divider. Diode 50 is connected such that it becomes conductivewhen the positive voltage at the junction of resistors 47 and 48 withrespect to ground becomes greater than the voltage developed acrosscathode resistor 21.

The lock-out circuit is a bridge circuit that includes diode rectifiers51 and 52 and resistors 53 and 54. The junction of resistors 48 and 49of the voltage divider is connected to one diagonal of the bridge at thejunction of diodes 51 and 52 so that when the diodes are nonconductive,the load-sensing circuits are eificiently disconnected from the input ofservo amplifier 29. The other diagonal of the bridge is connected inparallel with the coarse-loading control circuit and capacitor 36 towhich is applied direct-current voltage that is developed in response tothe operation of rate generator 27. When the plate circuits of the poweramplifier stage are being tuned and the rate generator is in operation,voltage of proper polarity is applied to the lock-out circuit to preventconduction through diodes 51 and 52.

The control system of this invention may obviously be applied to poweramplifier stages having different circuit arrangements for plate tuningand antenna loading. For example, rather than using a reactive L-networkfor coupling the antenna to the plate circuit, an. R.-F. trans- 45former for inductive coupling may be used, and coupling between itsprimary and secondary windings may be varied.

In the power amplifier stage, according to the accompanying schematicdiagram, the plate circuit is automatically tuned and the coupling tothe antenna is adjusted in response to change of frequency in thepreceding exciter. When the plate circuit is resonant at a difierentfrequency from that of the signal which is applied to the input circuit,the phase difference on the plate and grid departs from the usual phasedifference. In response to departure of phase from 180, phase detector24 develops a control voltage having a polarity dependent upon thedirection of departure. This control voltage is applied to servoamplifier 25 for operating servo-motor 26 in a direction dependent uponthe polarity of the control voltage. The servo-motor operates to adjustvariable capacitor 13 and variable inductor 12 until the plate circuitis tuned to the frequency of the signal that is applied to the inputcircuit. When the plate circuit is properly tuned, signal voltage on theplate is 180 out of phase with the signal voltage applied to the controlgrid, and the control voltage at the output of phase detector 24 becomesZero to stop operation of servo motor 26.

A rate generator 27 is operated by servo-motor 26 during the platetuning process to apply alternating-current voltage across the primaryof transformer 28. The voltage induced into the secondary winding oftransformer 28 is rectified by diode 35 for applying directcurrentvoltage across the coarse-loading control circuit that includespotentiometers 31 and 32 and across the lock-out circuit that includesdiodes 51 and 52. The lock-out circuit is responsive to the applicationof voltage thereto for isolating the fine-loading control circuits fromthe input circuit of servo amplifier 29. When the coarse-loading controlcircuit is unbalanced, control voltage is applied from the junction ofresistors 33 and 34 to the input of servo amplifier 29. Potentiometers31 and 32 of the coarse-loading control circuit are operatedsimultaneously by plate tuning servo motor 26 and loading servo motor39, respectively. Control voltage applied to servo amplifier 29 as aresult of operation of potentiometer 31 causes motor 30 to operatepotentiometer 32 in the required direction for maintaining the controlvoltage to servo amplifier 29 near zero. At the completion of the tuningcycle, variable capacitor 15 and variable inductor 14 have beenapproximately positioned by servo motor 30 for providing the properdegree of coupling between the antenna and the plate circuits. Whenservo motor 26 ceases operation in response to the plate circuit beingtuned to the proper frequency, direct-current voltage supplied from rategenerator 27 is removed from the coarse-loading control circuit and thelock-out circuit. Then the diodes 51 and 52 of the lock-out circuitconnect the fine-loading control circuit to servo amplifier 29.

After the plate circuit has been tuned, the fine-loading control circuitapplies control voltage to servo amplifier 29 to operate servo motor 30for exactly positioning variable capacitor 15 and variable inductor 14.When the signal voltage applied to control grid 18 of tube 11 is withinthe range of voltage for linear operation of tube 11, diode St} isnon-conducting and the control voltage for application to servoamplifier 29 is the function of only the ratio of the voltage on plate19 to the voltage on grid 18. Positive voltage on the arm ofpotentiometer 46 is developed by that rectifier which is connectedthrough coupling capacitor 41 to control grid 18, and negative voltageon the arm of potentiometer 40 is developed by that rectifier which isconnected through coupling capacitor 55 to plate 19. The resultingvoltage obtained from the comparison of positive and negative voltagesthat are applied to the voltage divider from the arms of thepotentiometers is present at the junction of resistors 48 and 49 that isconnected to the input of the setvolamplifier 29. When the amplifierstage is not properly loaded, a control voltage with polarity that isdependent upon the direction of departure from the ratio of the platevoltage to the control grid voltage, is applied to the-servo amplifier.Servo motor 30 operates until variable inductor 14 andvariable capacitor15 are positioned toreturn the voltage ratio to a predetermined valueand thereby to restore the control voltage to zero. Since adjustnientsof loading elements 14 and 15 and tuning elements 12 and 13 are notfully independent, the servo Systems for tuning and for loading mayoperate alternately until the reactive elements are adjusted preciselyfor resonance and for desired loading.

.Assume that diode 50 which is connected to cathode 17 .is disconnectedfrom the circuit and that input signal to grid 18 is gradually increasedin amplitude from within that range for linear operation of the tube toa value for a non-linear or class C operation. The ratio of platevoltage :to control grid voltage remains constant for signal changeswhile thetube is operating linearly, but for higher signal voltage, theratio changes when increases in plate current are non-linear withincreases of grid voltage. Since the plate current becomes limited forhigher input signals, the negative voltage that is derived from theplate circuit and is applied to the voltage divider in the fine-loadingcontrol circuit does notincrease as fast as the positive voltage that isderived from the grid circuit. Thedifterent rates of voltage changeproduce a control voltagehaving polarity for reducing the couplingbetween the, antenna and the plate circuit. In practice, the controlvoltage to servo amplifier 29 should remain constant for signals. of aparticular frequency so that the output impedancerernains the sameregardless of the amplitude of the input signal.

.To maintain output impedance constant for higher input voltages, thecontrol voltage which is derived from comparing voltage on plate 19 withvoltage on grid 18 is modified by connecting a tap of the voltagedivider through diode 50 to cathode 17. While diode 50 is conductive,change in fine-loading control voltage due to changejin voltage on thecontrol grid is small because the positive end of the voltage divider iseffectively clamped through diode 50 to cathode 17. The diode becomesconductive when the voltage at the junction of resistors 47 and 48becomes greater than the voltage on cathode 17 and the diode therebyconnects resistor 21 in parallel with that portion of the voltagedivider that is connected through potentiometer 46 to ground. Resistanceof the cathode circuit is substantially less than the resistance ofresistor 47 and potentiometer 46. Therefore, when a strong input signalis applied to control grid 18, control voltage is derived namely fromthe ratio of the voltage on the plate to the voltage on the cathode.

Since the transition from the non-conductive to the conductive state ofthe diode is gradual, the transition from grid voltage to cathodevoltage for developing a control voltage is also gradual to conform withthe gradual change in the operating condition of the tube. Throughproperselection of values in the voltage di viderand cathode circuits, thecontrol voltage at the junction of resistors 47 and 48 remains constantfor signal of wide amplitude at a selected frequency. The negativevoltage that is applied-to the voltage divider for different operatingconditions of the tube is proportional to the flow .of signal current inthe plate circuit; and obviously for strong input signal, the positivevoltage that is applied to the divider is proportional to the platecurrent which flows'through cathode resistor 21 The control circuits ofthis invention stabilize automatic loading servo systems for widevariations and amplitude-of input signals. The control circuits havebeen applied to single side-band transmitting systems in which inputsignal to the power amplifier stage at times exceeds themaximum signalallowable for linear operation.

.Although this invention has been described with respect to a particularembodiment thereof, it is to be understood that changes andmodifications may be made therein which are within the full and intendedscope of the invention as described in the appended claims.

What is claimed is:

1. In a power amplifier stage having an input circuit and an outputcircuit, a load sensing circuit having voltage comparator means and loadcurrent sensing means for controlling load impedance of said outputcircuit, said voltage comparator means being connected between saidinput circuit and said output circuit, servo means electricallyconnected to said voltage comparator means, said voltage comparatormeans responsive to a change in ratio of signal voltage on said outputcircuit to the signal voltage on said input circuit for applying directcontrol voltage to said servo means, variable reactance elements in saidoutput circuit, said servo system operating in response to applicationof control voltage to adjust said variable reactance elements until apredetermined ratio is obtained between the voltage on said outputcircuit and voltage on said input circuit, said load current sensingmeans being electrically coupled to said output circuit and to saidvoltage comparator means, said load current sensing means responsive tosignal voltage on said input circuit exceeding a predetermined value forchanging said direct control voltage in proportion to change in currentin said output circuit, said voltage comparator means and said loadcurrent sensing means cooperating to maintain adjustment of saidreactance elements thereby to allow a greater ratio of voltage in saidoutput circuit to voltage in said input circuit than said predeterminedratio that is obtainable by operation of only said voltage comparatormeans.

2. In a power amplifier stage including an electron vacuum tube havingat least a cathode, a control grid, and a plate; an input grid circuitconnected to said control grid, an output plate circuit connected tosaid plate, said plate circuit including reactance elements that areadjustable for determining the impedance thereof, a servo systemoperable for adjusting said reactance elements to obtain requiredimpedance in said plate circuit at ditferent frequencies, a load sensingcircuit having in combination a voltage comparator circuit and a loadcurrent sensing circuit for controlling the operation of said servosystem, said voltage comparator circuit connected to said grid and platecircuits and to said servo system, said voltage comparator circuitresponsive to a change in ratio of voltage on said plate circuit to thevoltage on said input circuit for applying direct control voltage tosaid servo system, said servo system operating in response to change ofvoltage from said voltage comparator circuit to adjust said variablereactance elements until a predetermined ratio is obtained between thevoltage on said plate circuit and voltage on said grid circuit, saidload current sensing circuit being electrically coupled to said platecircuit and to said voltage comparator circuit, said load currentsensing circuit responsive to voltage on said grid circuit exceeding apredetermined value for applying voltage to said comparator circuit inproportion to current flow in said plate circuit, said voltagecomparator circuit and said load current sensing circuit cooperating tomaintain adjustment of said reactance elements thereby to allow agreater ratio of voltage in said plate circuit to voltage in said gridcircuit than said predetermined ratio that is obtainable by operation ofonly said voltage comparator circuit.

3. In a power amplifier stage including an electron vacuum tube havingat least a cathode, a control grid, and a plate; an input grid circuitconnected to said control grid, an output plate circuit connected tosaid plate, said plate circuit including reactance elements that areadjustable for determining the impedance thereof, a servo systemoperable for adjusting said reactance elements to obtain requiredimpedance in said plate circuit at difierent frequencies; thecombination of a voltagetcom- '2' parator circuit and a load currentsensing circuit for controlling the operation of said servo system, avoltage divider having end terminals and first and second intermediatetaps, said voltage comparartor circuit including first and secondrectifier circuits having individual input circuits connected to saidgrid circuit and to said plate circuit respectively and having outputcircuits connected individually to said end terminals of said voltagedivider, said servo system having an input control circuit connected tosaid first intermediate tap, said load current sensing circuit includinga resistor and a rectifier, a plate return circuit including saidresistor and said cathode, said rectifier being connected between saidresistor and said second intermediate tap, said rectifier conductingcurrent in response to signal voltage on said grid circuit exceeding apredetermined value, said voltage comparator circuit and load currentsensing circuit cooperating to develop control voltage for said controlcircuit, and said servo system operating in response to the applicationof said control voltage to maintain constant impedance in said outputcircuit for extreme values of plate current.

4. In a power amplifier stage having an input circuit and an outputcircuit, the combination of a voltage comparator circuit and anamplifier load current sensing circuit for controlling the loadimpedance of said output circuit, said voltage comparator circuitincluding first and second voltage sensing circuits, a resistive voltagedivider having first and second intermediate taps, said first voltagesensing circuit being connected between said input circuit and one endof said resistive voltage divider for applying direct voltage ofpredetermined polarity to said divider, said second voltage sensingcircuit being connected between said output circuit and the opposite endof said divider for applying direct voltage of opposite polarity fromsaid predetermined polarity to said divider, a variable resonant elementin said output circuit, servo means coupled to said first intermediatetap of said voltage divider, said servo means operating in response tovariation from a predetermined ratio of signal voltage on said outputcircuit to signal voltage on said input circuit, said servo meansoperating to adjust said variable resonant element for obtaining saidpredetermined ratio, said load current sensing circuit being connectedto said output circuit and to said second intermediate tap on saidvoltage divider, said load current sensing circuit responsive to gridvoltage change to plate current change exceeding a predetermined valuefor changing the voltage on said first intermediate tap, and saidcombined control circuits efiective in maintaining the load impedance ofsaid output circuit substantially constant for wide variations inamplitude of signal on said input and output circuits.

5. in a power amplifier stage including an electron tube having at leasta cathode, a control grid, and a plate; an input circuit connected tosaid control grid, an output circuit connected to said plate, thecombination of a voltage comparator circuit and a load current sensingcircuit for controlling the impedance of said output circuit, a servosystem with a control circuit electrically connected to said comparatorcircuit, said comparator circuit being cormected to said input andoutput circuits, said load current sensing circuit including animpedance element and a rectifier, said impedance element beingconnected to said cathode in the plate return circuit of said tube, saidrectifier being connected between said impedance element and saidcomparator circuit, said rectifier being non-conducting in response toapplication of higher voltage from said impedance element than from saidcomparator circuit, reactance elements in said output circuit, saidservo system operating to adjust said reactance elements thereby tochange the ratio of voltage on said output circuit to the voltage onsaid input circuit, said voltage comparator circuit responding to changein ratio of voltage on said output circuit to voltage on said inputcircuit for providing a change 7 if in voltage in the control circuit ofsaid servo system, said servo system operating in response to operationof only said comparator circuit to maintain said voltage ratio constant,said rectifier becoming conductive in re- 7 sponse to the voltage tosaid rectifier from said comparator circuit becoming greater than thevoltage from said impedance element when signal voltage on said inputcircuit is high, said load current sensing circuit responsive to changein plate current of said tube to change the voltage on said comparatorcircuit thereby to change voltage on said control circuit, and saidservo system responding to the combined operation of said voltagecomparator circuit and said load current sensing circuit to change saidvoltage ratio as required for maintaining the impedance of said reactiveelements substantially constant for different operating conditions ofsaid tube over wide variations in voltages on said input and outputcircuits.

6. In a power amplifier stage having an input circuit and an outputcircuit, first and second adjustable reactive elements in said outputcircuit, tuning means operable to adjust said first reactive element fortuning said output circuit to a desired frequency, servo means fortuning said second reactive element for determining the loading in saidoutput circuit, load control circuits responsive to a change of loadingin said output circuit for developing a control voltage, a lock-outcircuit, said load control circuits being connected through saidlock-out circuit to said servo means, said servo means responsive toapplication of control voltage from said load control circuits formaintaining impedance of said output circuit constant regardless offrequency and amplitude of signal in said output and said inputcircuits, a voltage generator coupled to said tuning means and connectedto said lockout circuit, said voltage generator responsive to operationof said tuning means to apply voltage to said lock-out circuit, and saidlock-out circuit responsive to application of voltage from saidgenerator to isolate electrically said load control circuit from saidservo amplifier.

7. In a power amplifier stage having an input circuit, an outputcircuit, and servo means with control circuits, said control circuitsincluding a bridge circuit having a series pair of diode rectifiers,said output circuit including impedance matching elements, said servosystem operating in response to application of control voltage to saidcontrol circuits for adjusting said impedance matching elements, avoltage comparator circuit connected to said input circuit and to saidoutput circuit for developing a control voltage that is a function ofthe ratio of the voltage on said output circuit to the voltage on saidinput circuit, said voltage comparator circuit being connected to saidcontrol circuits for applying control voltage thereto, adjustableelements for tuning the output circuit of said stage, a rate generator,means for simultaneously tuning the output circuit of said stage and foroperating said rate generator, said comparator circuit being connectedto the junction of said rectifiers of said control circuits, means forconnecting said rate generator across said series rectifiers, and saidrectifiers becoming non-conducting in response to the operation of saidrate generator to isolate said comparator circuit from said servosystem.

8. In a power amplifier stage having an input circuit, an outputcircuit, and servo means with control circuits; impedance matchingelements in said output circuit, said servo system operating in responseto application of control voltage to said control circuits for adjustingsaid impedance matching elements, a voltage comparator circuit connectedto said input circuit and to said output circuit for developing a firstcontrol voltage that is a function of the ratio of the voltage on saidoutput circuit to the voltage on said input circuit, a lock-out circuit,said voltage comparator circuit being connected through said lock-outcircuit to said control circuits for applying said first control voltagethereto, adjustable reactive elements for tuning the output circuit ofsaid stage, means for simultaneously tuning said output circuit and fordeveloping a second control voltage for application to said lock-outcircuit, said lock-out circuit operating in response to application ofsaid second control voltage to isolate said comparator circuit from saidservo system during tuning of said output circuit.

9. In a power amplifier stage having an input circuit and an outputcircuit, first and second adjustable reactive elements in said outputcircuit, tuning means operative for adjusting said first reactiveelement for tuning said output circuit to a desired frequency, servomeans for tuning said second reactive element for determining theloading in said output circuit, control circuits including a voltagecomparator circuit, a load current sensing circuit, and a lock-outcircuit, said voltage comparator circuit being connected to said inputand said output circuits for developing a control voltage which is afunction of the ratio of the voltage on said output circuit to thevoltage on said input circuit, said load current sensing circuit beingcoupled to said output circuit for developing control voltageproportional to the current flow in said output circuit, said voltagecomparator circuit and said load current sensing circuit being connectedthrough said lock-out circuit to said servo means, said servo meansresponsive to application of control voltage from said voltagecomparator circuit and said load current sensing circuit for maintainingimpedance of said output circuit constant regardless of the frequencyand amplitude of signal in said output and said input circuits, avoltage generator coupled to said tuning means and electricallyconnected to said lock-out circuit, and said voltage generator operativein response to operation of said tuning means for disabling saidlock-out circuit to isolate said voltage comparator circuit and saidload current sensing circuit from said servo amplifier.

10. In a power amplifier stage having an input circuit, an outputcircuit, and servo means with control circuits; impedance matchingelements in said output circuit, said servo system operating in responseto application of control voltage to said control circuits for adjustingsaid impedance matching elements, a load current sensing circuitelectrically coupled to said output circuit for developing a firstcontrol voltage proportional to the current flow in said output circuit,a lock-out circuit, said load current sensing circuit being connectedthrough said lock-out circuit to said control circuits for applying saidfirst control voltage thereto, adjustable reactive elements for tuningthe output circuit of said stage, means for simultaneously tuning saidoutput circuit and for developing a second control voltage forapplication to said lock-out circuit, said lock-out circuit operating inresponse to application of said second control voltage to isolateelectrically said load current sensing circuit from said servo systemduring tuning of said output circuit.

11. In a power amplifier stage having an input circuit and an outputcircuit, said output circuit including first and second adjustablereactive elements, servo means for adjusting said second reactiveelements, a lock-out bridge circuit, a tine-loading control circuitresponsive to a change of loading in said output circuit for developinga control voltage, said fine-loading control circuit being connectedthrough said lock-out bridge circuit to said servo means, said servomeans responsive to application of control voltage from saidfine-loading control circuit for positioning exactly said secondreactive element, a coarse-loading control circuit connected to saidservo means, said coarse-loading control circuit including first andsecond variable voltage control elements connected in a bridgearrangement, a voltage generator connected to said lock-out bridgecircuit and to said coarse-loading control circuit, tuning means forsimultaneously adjusting said first reactive elements, operating saidvoltage generator and varying said first voltage control element, saidvoltage generator operating to apply voltage to said lockout bridgecircuit and to said coarse-loading control circuit, said lock-outcircuit operating in response to application of voltage from saidvoltage generator to isolate electrically said fine-loading controlcircuit from said servo means, said servo means operating in response toapplication of voltage to said coarse-loading control circuit toposition said second reactive element and to vary said second variablevoltage control element, said servo system operating to vary said secondvariable voltage control element until said bridge circuit arrangementis substantially balanced so that positioning of said second reactiveelement is a function of the positioning of said first reactive element,and said lock-out circuit electrically coupling said fine-loadingcontrol circuit to said servo means when said voltage generator isinoperative.

References Cited in the file of this patent UNITED STATES PATENTS2,376,667 Cunningham et al May 22, 1945 2,498,078 Harrison Feb. 21, 19502,498,340 Morrison Feb. 21, 1950 2,754,067 True et a1. May 8, 1956

